This shared instrument proposal is for purchase of a hybrid multi-collimator microSPECT/CT imaging system (MILabs MicroSPECT4CT), with a goal of high resolution, high sensitivity radionuclide targeted molecular imaging in small animals. The proposed state-of-the-art SPECT/CT imaging system will support the needs of multiple current NIH funded investigators at Yale University School of Medicine, from the Departments of Medicine, Diagnostic Radiology, Laboratory Medicine, and Pathology. The user group worked on a variety of projects, which would utilize multiple features of the camera including the proposed four different collimators. The 4 different collimators provide options for high resolutio and high sensitivity in both rat and moues models. Additionally, this system can acquire cardiac and respiratory gated images in addition to list mode data for high sensitivity dynamic imaging. The combined SPECT and CT unit provides inherent registration of functional SPECT images with high resolution and anatomical CT images for the purpose of image quantification. The CT images allow for correction of attenuation and partial volume errors associated with SPECT imaging, permitting absolute quantification of radiotracer uptake. These system features are critical for our program wherein we have a spectrum of NIH funded programs from assessment of mouse atria to dynamic flow measurements in the rat myocardium or lungs. To obtain FDA approval for an IND, this system can be used to evaluate in vivo targeted radiotracers for biodistribution, specific organ uptake and clearance kinetics. The merging of SPECT and CT data will also facilitate MIRD calculations for estimation of absorbed radiation dose. The registration of SPECT and CT data will also facilitate correlation of SPECT images with independently acquired PET and MR images as we moved towards multi-modality imaging projects. Thus, the proposed system would meet all of the needs of the investigators of this shared instrumentation grant, allowing for high resolution, high sensitivity imaging in small animals and absolute quantification of radiotracer uptake.